The invention relates to methods, to apparatuses and to a system for asynchronous spread-spectrum communication over a shared channel.
In particular, the invention applies to the field of satellite communication between mobile user terminals and one or more gateway through a satellite link.
In the near future satellite communications, in particular in the S-band, are expected to be used in settings other than the baseline one-way scenario. In particular, a new range of applications can take advantage of the S-band assigned to Earth-to-space communications in addition to the space-to-earth direction. This implies implementation of a two-way communications protocol, fully integrating the one-way broadcast protocol, and the development of a new system architecture, and of the related subsystems.
In any case, future applications will be aimed at mobile terminal use, taking advantage of the possibility of implementing small antennas in S-band. This implies that such two-way applications would take advantage of the integration with positioning systems (GPS/Galileo) enabling location-based information and services to the users. The design of a simple and low-cost yet high performance mobile satellite messaging return link represents a technical challenge.
A communication system according to the invention aims at providing broadcast-enabled integrated two-way communications, compatible with the IP protocol and leveraging as much as possible existing communications and broadcast standards, for the provision of non real-time messaging services from and/or to a large set of terminals (of the order of millions).
The invention is mainly directed to non-real-time messaging (data collection or short text messaging). In such an application, individual messages have a typical length of a few hundreds of bytes, and a low bit rate (e.g. a few kbps). The delivery delay should be from a few seconds to a few minutes (even more if the terminal is not in visibility of the satellite). The activity factor is estimated in a few tens of Kbytes per user per day (e.g. 100 messages of 100 bytes=10 KB), i.e. a very low one.
Such a low duty-cycle makes efficient implementation of the return link (or uplink) challenging, because:                closed loops for timing synchronization, power control, access control (Demand Assignment Multiple Access—DAMA), etc. . . . cannot work properly;        slotted random access solutions such as Slotted-Aloha or the more recently proposed Contention Resolution Diversity Slotted Aloha (CRDSA)—see document EP1686746—should also be avoided as they would require an unacceptable signaling overhead.        
Slotted Aloha (SA) protocols are used in TDMA (Time Division Multiple Access) systems with low efficiency and reliability. The MAC (Medium Access Control) layer throughput is pretty poor for SA (Throughput T=10−3 b/s/Hz for a packet loss ratio—PLR—of 10−3). Higher throughput may be achieved relaxing the PLR requirement and thus calling for packet retransmissions. Terminal burst synchronization is very inefficient for large number of terminal with (very) low transmission duty cycle like it is the case in the envisaged applications. In fact, burst slot synchronization requires an unacceptable signaling overhead in both the forward and return links. Finally, for SA the terminal EIRP (Effective Isotropically Radiated Power) requirement is related to the aggregated data rate of the TDMA multiple access scheme, rather than to the single terminal bit rate, and this penalizes low-cost terminal solutions.
Slotted Aloha protocols are described e.g. by “ALOHA Packet Systems with and Without Slots and Capture”, ARPANET System Note 8 (NIC11290), June 1972.
The paper by G. L. Choudhury and S. S. Rappaport, “Diversity ALOHA—A Random Access Scheme for Satellite Communications”, IEEE Trans. on Comm. Vol. COM-31, No. 3, March 1983, pp. 450-457 describes an enhanced version of Aloha known as Diversity Slotted Aloha (DSA). Document EP1686746 and the paper by E. Casini, R. De Gaudenzi, O. del Rio Herrero, “Contention Resolution Diversity Slotted ALOHA (CRDSA): An Enhanced Random Access Scheme for Satellite Access Packet Networks”, IEEE Trans. on Wireless Comm., Vol. 6, No. 4, April 2007, pp. 1408-1417 describe a further improvement of the Aloha protocol, known as Contention Resolution Diversity Slotted Aloha (CRDSA). CRDSA allows increasing the MAC throughput by an order of magnitude with respect to standard SA without degrading the PLR.
Spread Spectrum Aloha (SSA), also called Spread Aloha, is an alternative random access protocol described in the paper by O. del Rio Herrero, G. Foti, and G. Gallinaro, “Spread-spectrum techniques for the provision of packet access on the reverse link of next-generation broadband multimedia satellite systems”, IEEE Journal on Sel. Areas in Comm., vol. 22, no. 3, pp. 574-583, April 2004. SSA shows potentially interesting features as it provides a higher throughput capability than SA or CRDSA for the same PLR target under equal power multiple access conditions and using powerful physical layer FEC (Forward Error Correction), i.e. of the order of G=0.45 b/s/Hz for a packet loss ratio of 10−3). Furthermore SSA allows operating in a truly asynchronous mode. Spread Aloha terminal EIRP is in principle linked to the single user data rate although extra power is required to combat the CDMA self-noise. Also from this point SSA of view provides advantages compared to SA.
However, the main drawback of SSA is its high sensitivity to multiple access carrier power unbalance, disrupting the throughput of the scheme (e.g. a lognormal carrier power standard deviation of 3 dB can diminish the throughput by several orders of magnitude).
The basic principle of the Spread-Aloha scheme is the following: when a satellite terminal has a packet to transmit, it picks up at random one spreading sequence among a predetermined set of sequences, and one possible spreading code phase, and transmits it. The transmit burst spreading sequence code timing randomization is particularly important in slotted spread Aloha and requires a significantly higher number of spreading sequences compared to unslotted spread Aloha to achieve similar performances. The number of spreading sequences used in the system has a direct impact on the gateway burst demodulator complexity (i.e. on the number of correlators required).
An important feature of the SSA scheme is that the throughput grows linearly with the channel load until a breakdown point is reached. This behavior can be explained by the fact that the packets are successfully decoded until the multiple access channel signal-to-noise plus interference ratio (SNIR) at the gateway is above the physical layer threshold. When the SNIR becomes lower than a threshold values, the packets can not be recovered anymore and the throughput collapses. This behavior is verified if signals emitted by all the users arrive at the gateway burst demodulator with equal power; otherwise the actual system behavior will deviate from this simple model.
Document U.S. Pat. No. 5,537,397 describes a Spread Aloha scheme wherein multiple transmitters transmit data signals using identical spreading codes. A single matched filter of a receiver receives all the signals. A broadcast timing control signal retards or advances timings of individual transmitters to offset the interleaved signals. A subtracting circuit subtracts first and strongest signals until a single signal remains, and then reinserts the subtracted signals in the receiver. The identical code spreading sequence used in all the transmitters and in the matched filter is selected from a specific class of codes known as maximum length shift register sequences in a length equal to 2n−1 for integer values of n. The document mentions the possibility of using successive interference cancellation (SIC) to increase throughput, but no practical SIC solution for packet mode operation are proposed.
Document U.S. Pat. No. 5,745,485 describes a further improvement of a SSA scheme, comprising multiple-access signal detection by using a small number of different spreading signals. The spreading sequence is selected depending on a property of the signal being transmitted, rather than on the transmitter as in Code Division Multiple Access (CDMA). Multiple transmitters can use pilot signals and transmit the multiple data signal with the selected spreading sequences. A hub station receives the multiple data signals and detects the multiple data signals with matched filters or correlators matched to the code spreading sequences. Outputs of the matched filters or of the correlators can create control signals for offsetting the transmitted data signals by advancing or retarding the transmission time of the multiple data signals from the multiple transmitters.
Document U.S. Pat. No. 6,625,138 relates to a data transmission method used in a CDMA-type radio system. A base station and terminal equipments exchange data at least in a packet switched mode, and a terminal equipment transmits to the base station on a random access channel a random access signal comprising at least a preamble and a data part multiplied by a spreading code. A predetermined set of spreading codes and signature sequences are stored in the terminal equipment, and each signature sequence determines one spreading code. The terminal equipment selects one signature sequence by a random process from the set of signature sequences and adds the selected signature sequence to the preamble of the random access signal. Further, the terminal equipment uses the spreading code corresponding to the selected signature sequence in the data part of the random access signal. Interference cancellation is performed at the base station according to the signature sequence of the preamble of the received random access signal, such that at least the interference caused by the received data part is eliminated from at least one other received signal in order to improve detection. Like above-cited document U.S. Pat. No. 5,537,397, this document fails disclosing practical solution for performing SIC with bursty transmission.
Document U.S. Pat. No. 7,065,125 describes a multiple access communication technique wherein a multitude of transmitters communicate with receivers using direct sequence spread spectrum signaling. The direct sequence codes are reused by a large number of simultaneous transmitters, so the system is named Code Reuse Multiple Access (“CRMA”). This reuse method requires only a small number of spreading codes relative to the number of simultaneous transmitters, and can use as few as one code for all the users. The direct sequence codes are not required to have special properties such as maximal length. The lengths of the spreading codes employed are not necessarily related to the bit or symbol interval. CRMA can be implemented on a Paired Carrier Multiple Access (“PCMA”) system with or without a novel receiver structure which is also described by the document.
The paper from Xiang Feng, Yan Li, Guangguo Bi, “A CDMA-slotted ALOHA broadband system for multiservices”, IEEE 1998 International Conference on Universal Personal Communications, ICUPC '98, Florence, Italy 5-9 Oct. 1998, Volume: 2, pp. 1131-1135, shows that CDMA can offer significant advantages in wireless environments, especially when large capacity and wide range of service rates must be supported. This paper proposes a CDMA-slotted ALOHA system, in which all transmitters use the same PN sequence but with different chip phases and packets can be captured and received because of the autocorrelation property of the PN sequence. Analysis and simulation results show that the maximum channel throughput of this system is much greater than conventional SA systems and multiple services can be supported with guaranteed QoS (Quality of Service).
The paper from Y. Tadokoro, H. Okada, T. Yamazato, A. Katayama, A. Ogawa, “A new packet detection scheme in CDMA unslotted ALOHA system with successive interference cancellation”, IEEE Global Telecommunications Conference, 2001, GLOBECOM '1 Nov. 2001, San Antonio, Tex., USA, Volume: 5, pp. 3173-3177, outlines that packet detection is one of the most important problems in packet communication systems. In a CDMA Unslotted ALOHA system, multiple access interference (MAI) makes the performance of the packet detection worse. To reduce the effect of MAI, the authors propose a new packet detection scheme wherein Successive Interference Cancellation (SIC) is applied. The packet's signal is detected after the cancellation of MAI using SIC. This proposed scheme gives good performance of the packet detection. The paper is based on the very stringent assumption of ideal power control. Moreover, it is assumed that each user terminal uses a different, unique sequence.
In the papers from Schelegel et al.:                P. Kota, C. Schlegel, “A wireless packet multiple access method exploiting joint detection”, IEEE International Conference on Communications, 2003, ICC '03, 11-15 May 2003 Volume: 4, pp. 2985-2989; and        C. Schlegel, R. Kempter, P. Kota, “A novel random wireless packet multiple access method using CDMA”, IEEE Transactions on Wireless Communications, June 2006 Volume: 5, Issue: 6, pp. 1362-1370;        
a novel packet-based multiple access scheme for connectionless, uncoordinated random channel access is proposed. Random packet CDMA, or RP-CDMA, utilizes a novel packet format which consists of a short header and a data portion. Each header is spread with a unique spreading code which is identical for all users and packets, while the data portion of each packet is spread by a randomly chosen spreading sequence. The receiver operates in two stages: header detection and data detection. For header detection a conventional spread spectrum receiver is sufficient. Headers are spread with a large enough processing gain to allow detection even in severe interference. The data portion is decoded with a sophisticated receiver, such as a multiuser detector, which allows for successful decoding of overlapping active packets. It is shown that the RP-CDMA system is detector capability limited and that it can significantly outperform spread ALOHA systems whose performance is limited by the channel collision mechanism. RP-CDMA also experiences a much smaller packet retransmission rate than conventional or spread ALOHA, and provides better spectral efficiencies.
Throughput of random-access schemes of the “ALOHA” family are strongly dependent on the channel load. Therefore, it is known from prior art to implement an emission-control algorithm at the user terminal level in order to avoid channel congestion. See, for example, Simon S. Lam and Leonard Kleinrock, Packet-Switching in a Multi-Access Broadcast Channel: Dynamic Control Procedures, IEEE Trans. on Commun., Vol. COM-23, September 1975 and document US 2003/0133409.
Document WO 2007/051111 describes a method to mitigate the effect of multipath interference in a CDMA base station. This method comprises transmitting replicas of a given sub-packet which are repeated and soft combined until the information is correctly received at the base station. This approach can only be followed in a terrestrial system which benefits from fast base station feedbacks (acknowledged/not acknowledged) but it is not applicable at all to a satellite system.
Document WO 02/33838 discloses a receiving method comprising parallel interference cancellation. This method is based on fast user terminal to base station feedback which allows stopping packet retransmission when the packet has been successfully corrected. Therefore it is not applicable to satellite mobile networks due to the large propagation delay making the acknowledged/not acknowledged feedback too slow.